But before to do this I would suggest you prepare the old surface first

1, Clean well with water
2, rough up the exposed glassed areas with a very rough file
3, clean with acetone

Steps 1 - 3 should ensure a good physical and chemical bond.

If you have some volume to return (say on the bottom of the skegs etc) when mix up the first batch with micro ballons (the website quoted will know what these are - but they are exactly what they sound like) or chopped up glass matt, this will allow the gelcoat to be formed into more robust shapes (and will be almost bullet proof afterwards)

When applying the gelcoat, it must be sealed in order to 'go off' properly. The best method by far is to use Cooking (tin) foil.

When filling small 'dinks' or holes, clean as above, fill the hole with Gelcoat and then cover with smoothed out tin foil - if done properly, you will need the minimum sanding afterwards to get a good finish.

When repairing skegs etc, the tin foil can be used to shape the mixture before it 'goes off'

Read the instructions carefully, they should contain curing times for various temps.

When sanding back (you should always put too much on, even when using the foil method) you will need to got from

a course file (depending on volume to remove)
Fine file
80 grit wet and dry
160 grit w/d
400/600 grit w/d
and finally 1000 grit w/d

Good luck